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Babies born to women who work later in pregnancy tend to be lighter than those whose mothers quit sooner, say researchers who compared the effect to smoking.

The U.K. researchers studied data from two large databases in Britain and the U.S. to look at how smoking compromises birth weight. They also estimated the impact of when a mother stops working, which they said hasn't been previously considered.

Babies considered to have low birth weight ar those weighing less than 2,500 grams. (Erik de Castro/Reuters)

"Smoking during pregnancy reduces birth weight and fetal growth, while taking maternity leave or stopping work up to three months before birth improves both outcomes," economics Prof. Marco Francesconi of the University of Essex and his co-authors concluded in the July issue of the Journal of Labor Economics.

"The effects of smoking are usually larger for younger or lower educated women, while work interruptions are found to be more significant for older mothers."

Francesconi said he and his colleagues were surprised at the effect, which might have been overlooked before. Normal birth weight ranges between 5.5 pounds to 9.9 pounds.

In the study, smoking was associated with a reduction in birth weight of between 4.9 ounces and 5.6 ounces (140 to 160 grams). The size of the associated effect of stopping work was about half that, an increase of nearly 2.5 ounces to about 2.8 ounces (70 to 80 grams) in birth weight.

Low-birth weight babies are at increased risk for serious health problems and disabilities, March of Dimes says.

Francesconi speculated that fatigue and stress might harm the nutritional environment of the fetus.

"If this is seriously taken on board, I would expect policy makers to discuss it more broadly and involve employers in the process, in terms of gender fair employment and promotion prospects and pay as well as in terms of incentives for child investment," he said in an email.

Healthy lifestyle key

The researchers weren't able to look at the type of work that women did, such as how physically demanding it is. If women changed their behaviour after they ceased working could also make a difference, said Prof. Kristi Adamo, a research scientist at the Children's Hospital of Eastern Ontario Research Institute in Ottawa. Adamo is investigating predictors of child birth weight.

"In my opinion, healthful maternal behaviours trump all else," Adamo said. "If you continue to work while engaging in a healthy lifestyle there is likely not a problem, however, if one's work environment is not conducive to overall physical and mental well-being, then clearly an early exit would be beneficial for the developing fetus and mom as well."

Adamo said the study explores an interesting perspective. But it's limited since the researchers weren't able to account for variables like body mass index before pregnancy and gestational weight gain that are important for fetal growth and birth weight.

Complications in pregnancy such as gestational diabetes, which contribute to higher birth weight, could be a reason why women stop working early, she added.

Adamo advised that:

Women enter pregnancy at a healthy weight.

Follow gestational weight gain guidelines.

Eat healthy and not excessively.

Exercise regularly at a moderate intensity.

Limit exposure to highly stressful situations when possible.

In the study, birth weight of babies born to women under the age of 24 was not affected by working time. The researchers took the child's sex and whether it was a first, second or third pregnancy into account in their statistical analysis.

In some parts of the survey, women were asked to recall birth weights, which they might not have been able to do accurately.